1. Field of the Invention
The present invention relates generally to the recording of high frequency digital data. More specifically, the invention relates to the improvement of bit error rate (BER) performance in data recorded and played back from a storage medium such as a magnetic tape, magnetic disk, optical disk, or an equivalent thereof.
2. Description Relative to the Prior Art
The ubiquitous digital computer is, of course, available in the marketplace with various data handling capacities. Even with a large scale so-called mainframe computer, there is a need for peripheral equipment for temporary or long term storage of digital data. With the ever increasing speed of digital systems, it is necessary that such peripheral equipment handle data at a speed compatible with the data-handling capability of a master computer. It is also necessary that the storage medium of the peripheral equipment store data with a high packing density, to limit the space required for huge data quantities.
A high packing density means that the recording medium must be capable of handling very short wavelength signals. Imperfections in the recording surface of a peripheral storage medium can play an important part in limiting the data-handling capability of peripheral equipment.
At a moderate data packing density, a surface defect can cause an isolated bit or a few isolated bits to be in error. These errors can usually be corrected by a data handling algorithm known in the art for example Reed Solomon. With a higher packing density however that same surface defect may cause a burst error. Thus, a more powerful error correcting algorithm is required. Unfortunately, short (one or two bit) channel margin related errors that can occur over microscopically small localized regions of the storage medium are detrimental to a moderately powerful error correction algorithm designed to handle burst errors. A very powerful error correcting code must be employed to handle both types of errors. Such a code, however, implies extensive calculations with an attendant increase in signal processing complexity, decoding speed and delay time; thus, it has not been practical to incorporate an error correction technique which can efficiently handle all of the various type of errors caused by a peripheral storage medium.
With small defects of the type commonly found in high-quality magnetic tape, for example, a digital tape recorder characteristically exhibits on the order of one bit error in 10.sup.5 bits. With error correction circuitry, on the other hand, a modern digital tape recorder can be expected to have one bit error in 10.sup.9 bits. Such error-correction capability is adequate for some data-storage uses, e.g. for transmitting image data wherein optical image integration tends to mask completely minor data errors. For storage and retrieval of more general digital computer data, however, the general view is that not more than one bit error in 10.sup.11 or 10.sup.12 bits can be tolerated in a computer system currently planned for the near future.
With the recognition that an error correcting code has practical limitations, U.S. Pat. No. 4,506,306 discloses digital signal processing apparatus for improving the bit error rate (BER) performance of digital tape recorder playback data having missing bit(s) due to tape dropout(s). To that end, a bit stream of input data is interleaved with itself in such a way that the respective interleaved portions are displaced from each other, within a tape record track, by a distance corresponding to at least the statistical maximum size of a tape dropout. By continuously delaying one such interleaved playback portion relative to the other for a time sufficient to bring the two playback portions into sync with each other, the original bit stream may be reconstructed by toggling back and forth between the two playback portions each time a dropout is detected in either playback portion.
Although the apparatus of the aforementioned patent works well for its intended purpose, it suffers from a disadvantage in that it is limited to overcoming data errors of the type that are characterized by the complete absence of a data pulse. In other words, parity errors and other data errors associated with commonly encountered statistically distributed tape defects may not be correctable. A further limitation is that, due to the interleaving of a data bit stream with itself, maximum raw data rate is limited to one-half the rate that a signal processing channel of the tape recorder can reliably handle.